The present invention relates to filtering of optical signals and, in particular, discloses an optical filter having electronic control.
The utilization of optical fibre transmission in telecommunications systems has become increasingly important due to their high bandwidth capabilities. Recently, Dense Wavelength Division Multiplexing (DWDM) Systems have become a popular choice as a result of their increased bandwidth possibilities.
In complex DWDM systems and in optical fibre transmission generally it is often necessary to amplify and filter signals during transmission. The degree of filtering required is often variable. This is particularly the case in the situation of filtering signals amplified using erbium doped amplifiers as the spectral dependence of amplification of an erbium doped fibre is dependant upon input conditions.
It is an object of the present invention to provide for an electronically controllable optical filter device suitable for use in telecommunications systems.
In accordance with a first aspect of the present invention, there is provided an optical filter for providing frequency dependant filtering of an optical input signal, the filter including: an input wave guide; an output waveguide; a polarisation separation means; a variable polarisation rotation means; the variation being dependant on a first control input signal; and a birefringent element having variable birefringence, the variation being dependant on a second control input signal; wherein an optical input signal emitted from the input waveguide passes through the polarisation separation means, the variable polarisation rotation means and the birefringent element to the output waveguide so as to produce at the output waveguide an optical output signal having a frequency dependant upon characteristics determined by one of the control input signals.
The frequency dependant output characteristic preferably can include an overall extinction magnitude (difference between the maximum and minimum attenuation in the bandwidth of interest) determined by the first control input signal and a frequency varying attenuation characteristic determined by the second control signal. The frequency dependant output characteristic preferably can include a oscillatory frequency response having a tuning determined by the second control input signal.
The input and output waveguide can have ends which are spaced apart at a first end of the filter and the filter further can include: a reflection element spaced at a second end of the filter such that the polarisation separation means, the variable polarisation rotation means and the birefringent element are preferably between the reflection element and the waveguides, such that light emitted from the input waveguide passes through the polarisation separation means, the variable polarisation rotation means and the birefringent element before being reflected by the reflection element so as to again pass through the birefringent element, the variable polarisation rotation means and the polarisation separation means before being emitted towards the output waveguide.
Preferably, at least one focussing means for focussing light emitted from the input waveguide towards the output waveguide is also provided.
In one embodiment, the maximum level of attenuation of light passing from the input waveguide to the output waveguide at any particular frequency can be determined by variation of the first control input signal, and the frequency of the maximum level of attenuation can be determined by variation of the second control input signal.
The variable birefringence can be provided by thermal variation of the birefringent element in proportion with the level of the second control input signal. The thermal variation can be provided by a Peltier device attached to the birefringent element. Alternatively, the variable birefringence can be provided by an electro-optic element.
The variable polarisation rotation means can comprise a Faraday rotator providing a rotation in accordance with an applied magnetic field and the filter can be formed between the poles of an electromagnet poles including an aperture through which the device passes. The external magnetic field can be provided by an initial permanent magnet providing a magnetic field having tangential and perpendicular components to a face of the Faraday rotator, and a second electromagnet having a magnetic field substantially perpendicular to the face of the Faraday rotator.
In accordance with a further aspect of the present invention, there is provided a method of filtering an optical input signal in a frequency dependant manner, the method comprising the steps of: (a) separating substantially orthogonal polarisation states; (b) imparting a wavelength dependant polarisation transformation function to at least one of the polarisation states; (c) combining the outputs of the step (b) to produce a substantially polarisation independent frequency dependant filter response. The step (a) can comprise spatially separating orthogonal polarisation states.
In accordance with a further aspect of the present invention, there is provided a method of filtering an optical input signal in a frequency dependant manner, the method comprising the steps of: (a) emitting the light from a first optical waveguide; (b) substantially separating orthogonal polarisation states emitted from the first optical waveguide; (c) rotating the separated orthogonal polarisation states in a first direction; (d) for polarisation states having a predetermined angular profile, delaying one orthogonal polarisation state relative to a second so as to produce optical frequency dependant polarisation states; (e) rotating the delayed polarisation states in a second direction to produce rotated delayed polarisation states; and (f) for predetermined angular profiles, translating one polarisation state with respect to a second so as to combine the polarisation states having predetermined angular profiles at an output waveguide wherein light emitted from the input optical waveguide is attenuated in a frequency dependant manner.
The step (c) can comprise rotating the polarisation states to a degree as determined by a first input control signal. The step (d) can comprise delaying one orthogonal polarisation state relative to a second by an amount as determined by a second input control signal. The frequency dependant filtering characteristics are preferably determined by the first and second input signal.
In accordance with a further aspect of the present invention, there is provided an optical amplifier including an amplifying element and an optical equaliser comprising a plurality of concatenated optical filter elements each having frequency dependant attenuation characteristics which can be dynamically frequency and extinction tuned in order to optimise the optical equalisation of an optical signal transmitted by the optical amplifier, at least one of the optical filter elements including an input wave guide; an output waveguide; a polarisation separation means; a variable polarisation rotation means, the variation being dependant on a first control input signal; and a birefringent element having variable birefringence, the variation being dependant on a second control input signal; wherein an optical input signal emitted from the input waveguide passes through the polarisation separation means, the variable polarisation rotation means and the birefringent element to the output waveguide so as to produce at the output waveguide an optical output signal having a frequency dependant output characteristics determined by one of the control input signals.
In accordance with a further aspect of the present invention, there is provided an optical communications system including an optical element further including an amplifying element and an optical equaliser comprising a plurality of concatenated optical filter elements each having frequency dependant extinction which can be dynamically frequency and overall attenuation tuned in order to optimise the optical equalisation of an optical signal transmitted by the optical element, at least one of the optical elements including an input wave guide; an output waveguide; a polarisation separation means; a variable polarisation rotation means, the variation being dependant on a first control input signal; and a birefringent element having variable birefringence, the variation being dependant on a second control input signal; wherein an optical input signal emitted from the input waveguide passes through the polarisation separation means, the variable polarisation rotation means and the birefringent element to the output waveguide so as to produce at the output waveguide an optical output signal having a frequency dependant output characteristics determined by one of the control input signals.
In accordance with a further aspect of the present invention, there is provided an optical attenuation filter including: an input an output wave guide having ends spaced apart at a first end of the filter, a polarisation separation means for spatially separating polarisation states emitted from the input waveguide; a variable polarisation rotation means, the variation being dependant on a first control input signal; and reflection means for reflecting light emitted from the input waveguide towards the output waveguide; wherein an optical input signal emitted from the input waveguide passes through the polarisation separation means, the variable polarisation rotation means, is reflected by the reflection means back through the variable polarisation means and the polarisation separation means towards the output waveguide so as to produce at the output waveguide an optical output signal having a polarisation independent extinction determined by first control input signal.
In accordance with a further aspect of the present invention, there is provided an optical filter including a variable polarisation rotation means comprising a Faraday rotator providing a rotation in accordance with an applied magnetic field and the filter is formed between the poles of an electromagnet with one of the poles including an aperture through which the input and output waveguide passes.